ETA: And the winners have been announced: Congratulations neatmonster and sweet_lyri ! (Thank you velvetwhip for the heads-up.)
I know for a fact that the mods at btvsats20in20, otherworldlyric and slayerstillness have all extended deadlines on the current challenges:
** June 24th for btvs Round 10 (Scroll down to see all the wonderful entry sets thus far, the latest from sweet_lyri and spikesredqueen).
** June 19th for both OWL Challenge 203 and SS Challenge 46.